The Brno Dam Reservoir produces a hydropeaking flow regime which causes that only a part of the chironomid community found above the reservoir is able to adapt. The very extreme conditions found directly below the dam cause a reduction in the number of taxa, seasonal fluctuation of density with the highest values in the part of the streambed with the shortest time of exposition. The flow conditions in tailwaters stabilize very soon, and the chironomid community of the regulated site C shows a number of taxa comparable with the numbers at site A and even higher densities (up to 3,270 ind./m²).